Re: TOAST usage setting

> I tested EXTERN_TUPLES_PER_PAGE for values 4(default), 2, and 1:
> 
> 	4	15.596
> 	2	15.197
> 	1	14.6
> 
> which is basically a 3% decrease from 4->2 and 2->1.  The 
> test script and result are here:
> 
> 	http://momjian.us/expire/TOAST2/
> 
> shared_buffers again was 32MB so all the data was in memory.

Thanks for the test. (The test is for 1 row that is 100k wide.)

It is good. It shows, that we even see a small advantage in the
everything cached case.

What we don't have yet is numbers for whether EXTERN_TUPLES_PER_PAGE=1
substantially increases the toast table size for real life scenarios,
what happens in the worst case (~48% wastage compared to previous 12%),
and whether 1 row per page works well with autovacuum ?

The bad case (with EXTERN_TUPLES_PER_PAGE=1) is when most toast tuples
have a size over TOAST_MAX_CHUNK_SIZE_for_2+1 but enough smaller than a
page that we care about the wasteage. Maybe we can special case that
range.
Maybe determine (and lock) the freespace of any cheap-to-get-at non
empty page (e.g. the current insert target page) and splitting the toast
data there. 

Andreas